Internal blow molding process

Abstract

A method for producing in a cylindrical thermoplastic material an internal thread by blow molding the thermoplastic material in a mold in which the surface of the mold defining the outer face of the thread is at a negative angle of at least 5 degrees to the planes perpendicular to the axis of the cylindrical material.

What I claim as my invention: 
     
       1. A method of forming an internal thread on a blow-molded article, said method comprising the steps of providing a mold having an internal surface disposed about a central axis and profiled surfaces in a part of the internal surface with one of said profiled surfaces disposed on a negative angle of 5° to 15° to a plane perpendicular to said central axis; and   blow molding a thermoplastic material within the mold to cause the material to flow into and conform at an outer surface thereof with said profiled surfaces while forming an internal surface having a damped configuration, including an internal thread having a flat gripping surface opposite said one profiled surface sufficient for threadably gripping and sealing with an external thread on a container.   
     
     
       2. A method of forming an internal thread in a cylindrical thermoplastic material for threadably engaging with an external thread on a container, said method comprising the steps of providing a mold having profiled surfaces to define a finished internal thread, one of said profiled surfaces having a negative angle of 5 to 15 degrees relative to the path of the profile of the thread; and   blow molding a thermoplastic material into the profiled surfaces to cause the material to flow into and conform at an internal surface thereof with said profiled surfaces while forming an internal surface having a damped configuration, including an internal thread within the material having a flat gripping surface opposite said one profiled surface sufficient to threadably grip and seal with an external thread on a container.   
     
     
       3. The method of claim 1 in which said thermoplastic material has a Shore hardness of A70-85. 
     
     
       4. The method of claim 1 wherein said angle is 10 degrees.